FACTORS OF FOREST PRODUCTION. 107 



volume. The crop is ready for harvest when a 

 sufficient number of annual growths is accumu- 

 lated to make wood of useful size. This differs 

 according to the use to which the material is to be 

 put. 



A five to ten years' growth of some kinds might 

 suffice for hop and bean poles, for barrel hoops, 

 canes, and the like ; at fifteen to twenty years the 

 crop might furnish in addition some fence posts and 

 poles as well as firewood, especially if grown from 

 coppice. At fifty years some of the trees may 

 have in part accumulated sufficient size to furnish 

 bolts for the manufacture of carriage stock, hubs, 

 and spokes, or small cooperage and other articles 

 of small dimension, or even railroad ties and tele- 

 graph poles. But with most species which are 

 used to supply the large demands of the lumber 

 market, sizes fit for the sawmill are in the temper- 

 ate zones attained hardly in less than 75 to 100 

 years ; while most of the trees that are now cut 

 for that purpose nature has taken 150 to 200 years 

 and up to 500 years or more to produce. 



In addition to size, quality, too, is a function of 

 age, improving as a rule with increase in size. To 

 produce a sawlog which will furnish a sufficiently 

 large amount of clear boards free from knots, many 

 years must have elapsed to cover with annual 

 layers the stumps of branchlets of the younger 

 tree, which by the shading of neighbors were 

 killed and broken off by winds or otherwise. 



